Scientists say this blue zircon crystal is 4.4 billion years old, making it the oldest known chunk of our planet. The discovery is forcing a rethink of Earth's early geological history — and our planet's capacity to harbor life.

The zircon crystal, which was extracted in 2001 from a rock outcrop in Australia, measures a mere 200 by 400 microns — about twice the diameter of a human hair. To age it, scientists used two different techniques: atom-probe tomography (which identifies and maps individual atoms) and by determining the radioactive decay of uranium to lead in the sample. Individual zircon rocks provide the only physical evidence from the earliest phases of Earth's geological past.

To put the extreme age of this rock into perspective, the Earth itself formed as a ball of molten rock about 4.5 billion years ago. The presence of this crystal suggests our planet cooled down enough to form a solid crust a mere 100 million years later, and 160 million years after the formation of the solar system. The finding strengthens the "cool early Earth" hypothesis, where temperatures were low enough to sustain oceans — and perhaps life — earlier than assumed. The Hadean eon, it now appears, may not have been as harsh as we thought.

Because the Earth formed a solid crust 4.4 billion years ago, it may have been capable of sustaining microbial life about 100,000 years later (though no fossil evidence exists to support this assertion). To date, the oldest sample of microbial life dates back about 3.4 billion years.